Atomic Clocks


    The Critical Need For High Fidelity Atomic Clocks

    The Deep Space Atomic Clock, developed by NASA, may be the most stable atomic clock ever sent into space. But what exactly does it imply, and how do clocks relate to space navigation?

    • The planned launch date for a technological demonstration that may change the way humans explore space is June 24, 2019. 
    • The Deep Space Atomic Clock, developed by NASA's Jet Propulsion Laboratory in Pasadena, California, is a significant improvement over satellite-based atomic clocks that, for example, allow GPS on your phone.

    In the end, this new technology may allow spaceships to go to faraway places such as Mars on their own. But, first and foremost, what is an atomic clock? 

    What makes the Deep Space Atomic Clock unique is how it is utilized in space navigation. 

    What is the purpose of using clocks to travel in space?

    Navigators transmit a signal to a spacecraft to calculate its distance from Earth, which the spacecraft subsequently returns to Earth. 

    • Because the signal travels at a given speed, the time it takes to complete the two-way trip indicates the spacecraft's distance from Earth (the speed of light).
    • While it may seem difficult, most of us utilize this idea on a daily basis. It's possible that the food shop is a 30-minute walk from your home. 
    • You can calculate the distance to the shop if you know you can walk a mile in 20 minutes.

    Navigators can determine a spacecraft's trajectory: where it is and where it is going, by transmitting various signals and collecting several measurements over time.

    • Quartz crystal oscillators are utilized in almost all contemporary clocks, from wristwatches to satellites. 
    • When voltage is given to quartz crystals, they vibrate at a specific frequency, which is used in these devices. 
    • The crystal's vibrations work like a grandfather clock's pendulum, keeping track of how much time has passed.
    • Navigators require clocks with precise time resolution - clocks that can measure billionths of a second - to determine the spacecraft's location to within a meter.
    • Clocks that are very steady are also required by navigators. 

    "Stability" relates to how consistently a clock counts a unit of time; for example, the length of a second must be constant across days and weeks (to better than a billionth of a second).

    What are the connections between atoms and clocks?

    • Quartz crystal clocks aren't particularly steady by space navigation standards. 
    • Even the best-performing quartz oscillators may be off by a millisecond after just one hour (one billionth of a second). 
    • They may be wrong by a whole millisecond (one thousandth of a second), or 185 miles, after six weeks (300 kilometers). 
    • This would have a significant effect on determining the location of a rapidly moving spacecraft.

    To attain better stability, atomic clocks combine a quartz crystal oscillator with an ensemble of atoms. 

    After four days, NASA's Deep Space Atomic Clock will be off by less than a nanosecond, and after ten years, it will be off by less than a microsecond (one millionth of a second). 

    This is the equivalent of being one second off every ten million years.

    Atoms are made up of a nucleus (protons and neutrons) that is surrounded by electrons. 

    • On the periodic table, each element represents an atom with a specific number of protons in its nucleus. 
    • Although the number of electrons swarming about the nucleus may vary, they must all occupy distinct energy levels, or orbits.
    • An electron may ascend to a higher orbit around the nucleus after receiving a shock of energy in the form of microwaves. 

    To accomplish this leap, the electron must receive precisely the correct amount of energy - which means the microwaves must have a very particular frequency.

    • The energy needed to get electrons to shift orbits varies per element, but it is constant for all atoms of a particular element throughout the universe. 
    • For example, the frequency required to alter the energy levels of electrons in a carbon atom is the same for all carbon atoms in the universe. 
    • Mercury atoms are used in the Deep Space Atomic Clock; a different frequency is required to cause those electrons to shift levels, and that frequency will be constant for all mercury atoms.
    • "It's really the essential element for atomic clocks because the energy difference between these orbits is such a precise and stable number," said Eric Burt, an atomic clock scientist at JPL. 
    • "It's because of this that atomic clocks can outperform mechanical clocks."

    The ability to detect this constant frequency in a specific atom provides science with a universal, uniform time measurement. 

    • The number of waves that travel through a given location in space in a given unit of time is referred to as "frequency."
    • It is therefore feasible to estimate time by counting waves.
    • In reality, the frequency required to have electrons jump between two particular energy levels in a cesium atom determines the official measurement of a second.

    The frequency of the quartz oscillator is converted into a frequency that is applied to a group of atoms in an atomic clock. 

    • Many electrons in the atoms will shift energy levels if the calculated frequency is accurate. 
    • There will be much fewer electrons jumping if the frequency is wrong. 
    • This will establish whether and how much the quartz oscillator is off-frequency. 
    • The quartz oscillator may then be steered back to the proper frequency using a "correction" defined by the atoms. 

    The Deep Space Atomic Clock calculates and applies this kind of adjustment to the quartz oscillator every few seconds.

    What makes the Deep Space Atomic Clock special?

    Onboard the GPS satellites that circle the Earth, atomic clocks are employed, although even these need to be updated twice a day to counteract the clocks' inherent drift. 

    Those updates are provided by more reliable atomic clocks on the ground, which are enormous (typically the size of a refrigerator) and not built to withstand the physical rigors of space travel.

    NASA's Deep Space Atomic Clock is designed to be the most stable atomic clock ever flown in space, up to 50 times more reliable than the atomic clocks on GPS satellites. 

    • Mercury ions are used to produce this stability.
    • Ions are atoms that are not electrically neutral but have a net electric charge. 
    • Atoms are confined in a vacuum chamber in any atomic clock, and in certain of those clocks, atoms interact with the vacuum chamber walls. 
    • Changes in the environment, such as temperature, will induce comparable changes in the atoms, resulting in frequency inaccuracies. 
    • Because the mercury ions have an electric charge, they may be confined in an electromagnetic "trap" to avoid this interaction, enabling the Deep Space Atomic Clock to reach a new degree of accuracy.

    Such accuracy makes autonomous navigation feasible with little communication to and from Earth for missions traveling to distant destinations like Mars or other planets, which is a significant advance over how spacecraft are presently guided.

    General Atomics Electromagnetic Systems of Englewood, Colorado supplied the spacecraft for the Deep Space Atomic Clock. It is supported by NASA's Space Technology Mission Directorate's Technology Demonstration Missions program and NASA's Human Exploration and Operations Mission Directorate's Space Communications and Navigations program. The project is overseen by JPL.

    ~ Jai Krishna Ponnappan

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